It is known that methacrylonitrile can be produced by catalytic ammoxidation of isobutylene in the gas phase at elevated temperature, and a number of catalysts have been proposed for this purpose. For example, Japanese laid-open patent application No. 49719/1973 discloses a multiple metal oxide catalyst system whose basic components are molybdenum, bismuth, iron, and cobalt or nickel. However, although subsequent improvements have been made on various aspects of catalyst systems of this type, when they are employed for the ammoxidation of isobutylene or tert-butanol, the yield of desired methacrylonitrile drops markedly within a short period of time. This adversely affects the suitability of such catalysts for industrial use.
In Japanese laid-open patent application Nos. 17334/1980 and 17356/1980, the present inventors disclosed an improved method for producing methacrylonitrile by gas-phase ammoxidation of tert-butanol or isobutylene at elevated temperature by means of a catalyst containing molybdenum, bismuth and iron as basic components, and further containing a trace amount of at least one element selected from potassium, rubidium and cesium. However, while improvements have thus been made in product yield as well as in catalyst life and stability, the formation of carbon deposits on the catalyst has continued to be a problem, particularly since methacrylonitrile readily undergoes oxidative decomposition whereby such deposits are formed. This results in increased co-production of methacrolein which makes it more difficult to isolate and purify the desired methacrylonitrile and adversely affects the yield of the latter. Although it is possible to regenerate the catalyst by burning off the carbon deposits, this requires interruption of the ammoxidation reaction which is a serious economic drawback, particularly in cases where the process is intended for continuous operation, e.g., in a fluidized bed-type reactor.
Thus, there has been a long-felt need for and extensive studies have been made to develop an improved industrial catalyst and process for the commercial production of methacrylonitrile by ammoxidation of tert-butanol or isobutylene. In particular, an ammoxidation catalyst and process has long been sought which is suitable for use in a fluidized bed reactor, has high catalytic activity and high product selectivity, and has a long use life unhampered by the formation of carbon deposits on the surface of the catalyst.